To achieve high data capacity in data tape recorders, a high number of recorded tracks are desirable. For systems relying on longitudinal recording (recording along the length of the tape), knowing the position of one of the tape edges with a high accuracy is crucial. For years, this was achieved by making systems with very precise mechanical dimensions and narrow tolerances. However, as requirements increased, this method became more and more difficult (and expensive). It is therefore desirable to be able to detect the edge of the tape precisely without relying on a very expensive mechanical design.
U.S. Pat. No. 4,407,503, incorporated herein, teaches a method for detecting the edge of the tape by using the recording (read/write) head itself. This has proved to be a very efficient method used extensively, especially in the 1/4" tape cartridge industry, in order to effectively increase the number of recorded tracks. This can easily be verified by the following figures from the 1/4" tape cartridge industry: In 1984 the state of the art was 9 tracks across the tape. In just 7 years this had increased to 30 tracks without major improvements in the mechanical tolerances of the tape cartridges or the tape drives themselves.
In U.S. Pat. No. 4,407,503, the sensing of the edge of the tape is based upon writing a signal along the edge of the tape and then detecting this signal by a read head gradually moving from a position away from the tape towards the tape edge until it completely covers the edge of the tape (or vice versa: from a position known to be on the tape to a position off the tape). The writing and reading may be performed as one operation in a read-while-write mode (with a head containing both a read and a write element) or as a two pass operation where the head first writes along the edge of the tape and then reads the same signal during the next pass. It is possible to perform the write operation while moving a head 10 from a position 10A completely away from a tape 11 through positions 10B and 10C to a position 10D completely on the tape 11 (see FIG. 1) or (especially if the writing element W is much wider than the reading element R), the writing can be performed along a tape edge 12, while keeping the head 13 stable. In the latter case, the edge detection must be performed during a second pass when the head 12 is moved towards the tape 11 so that the read element can detect the signal along the tape edge 13. See FIG. 2.
When moving from a position off the tape 11 to a position on the tape 11, the output of the read head R will increase from almost zero level (noise) until a maximum value "M" when the whole read section of the head is covering the tape reading the recorded signal. This is shown in FIG. 3.
When designing an edge detection system, the system designer must pick a certain signal value as a triggering point ("pseudo edge"). This is referred to as "T" in FIG. 3. The designer may in theory pick any point on the curve of FIG. 3 as a reference point, though for many reasons it is common practice to pick T at somewhere between 15% and 30% of the maximum value ("M"=100%). If we assume that the maximum is 1 volt out of the read amplifier (M=1 volt), then T=20% means that the circuit is designed so that the electronics will trigger at 0.20 Volts. The designer will try to design the electronics to be very stable so that this triggering point "T" is not influenced by component variations, temperature changes, etc.
Once the triggering point "T" is selected in the design process, calculations and tests are used to determine the actual distance the read head has to travel on a typical drive and with a typical tape from the point where the head first touches the edge of the tape (approximately point "E" in FIG. 3) until it has reached point T. Once this distance is determined, the drive control system can move the head to any predetermined track position with a high degree of accuracy. Commonly a stepper motor is used to move the head. Therefore, all distances may be given as a specific number of steps of the stepping motor.
If we assume that the width of the read head section is equal to the distance travelled by the head in, for example 50 steps, then the distance "E" to "M" in FIG. 3 is equivalent to 50 steps and "E" to "T" is equivalent to 10 steps. FIG. 4 shows a typical track layout where the first track I.sub.0 is placed nominally 100 steps from the edge 15 of the tape 14, and the following tracks T.sub.1 and T.sub.2 have a center-to-center line distance L.sub.1 and L.sub.2 of nominally 80 steps.
In calculating the nominal distance from the edge to the point on the tape 14 where trigger point "T" is reached (set to 10 steps in the example above), the designer will make calculations and verify the result by using a "typical" drive and tape in a nominal environment. Although tape may vary considerably, this method has been quite acceptable to meet the requirements up to the levels used in the industry today. For example, tape specifications typically allow for a variation in the tape output of -35% to +50% from the defined nominal 100% level. If we use the numbers in the example above, this means that the triggering point T will vary approximately from 6.7 steps minimum (with a +50% tape) to approximately 15.4 steps maximum (with a -35% tape). See FIG. 5. If the distance from the edge of the first track is 100 steps, this change in tape will introduce a track position error of a maximum of approximately 5%. For most systems, this is acceptable.
To reduce the variation due to tape output tolerances, some designers have designed the electronics so that the read detection amplifiers have a very high amplification (gain). Therefore, the output from the read amplifier will increase very rapidly even at very low signal levels. The amplifier will saturate long before the nominal head output is reached. This will reduce the effect of tape output variations. See FIG. 6.
This method has the drawback that the noise in the system also will be amplified in the same way. Therefore, the triggering point must be set at a very high level, reducing the effectiveness of the method.
U.S. Pat. No. 4,977,468, incorporated herein, teaches another method to overcome variations in tape output by using two read heads. The signal from one of the read heads is compared by a comparator to the output signal from the other one. The operation starts by having both read heads covering a portion of the tape which contains a recorded signal. The heads are then gradually stepped away from the tape. As the first read head moves away from the tape, the read signal from this head will drop and the comparator circuit will trigger. Since the comparator is comparing the output signals from both read sections, variations in tape output will be the same for both outputs and therefore not influence the comparator (common mode rejection).
As mentioned, tapes used in the industry today may, according to the specifications, vary in output from -35% to +50%. These specifications are for the tape at the time of first recording. During use, tape output will be reduced, especially in areas which often pass over the read head.
For most tape systems, that means the Beginning Of Tape (BOT) area, which also is the area typically used to determine the edge of the tape. Therefore, on a used tape, output levels may be less than specified -35% from nominal value. Variations in temperature may reduce the level even more. This obviously makes edge detection less precise.
Until now, the method taught in U.S. Pat. No. 4,407,503 has met the requirements for track accuracy used in the industry, even with the variations in the tape outputs referred to above. However, as capacity requirements continue to grow, even more precise methods to find the edge of the tape is required, so that track densities can be increased further.
The method taught in U.S. Pat. No. 4,977,468 has so far been the only described method to further improve the edge sensing operation based upon U.S. Pat. No. 4,407,503. The method defined in U.S. Pat. No. 4,977,468 will improve the edge sensing tolerances; however, the cost of the dual read head design is significant and should be avoided if possible.